1. Field of the Invention
The present invention relates to a Raman amplification repeater which utilizes amplification of signal light by Raman scattering that occurs when an excitation LD is input into a transmission line fiber. In particular, the present invention relates to a Raman amplification repeater which enables expansion of a signal bandwidth and external control of Raman amplification.
2. Description of the Related Art
The recent advancement of wavelength multiplexing technology has allowed current transmission systems to multiplex a wavelength into many, generating the necessity of repeaters (e.g., submarine repeaters) with wider bandwidths than in the past. Raman amplification is a technique effective in achieving a wider bandwidth in repeaters, and studies have actively been addressed to develop a repeater based on this technique. This repeater, called a “Raman amplification repeater,” utilizes amplification of signal light by Raman scattering that occurs when an excitation LD in the 1480 nm bandwidth is input into a transmission line fiber.
Examples of such Raman amplification repeater include the ones disclosed in Japanese Patent Laying-Open (Kokai) No. 2001-15845 Official Gazette (Literature 1) and Japanese Patent Laying-Open (Kokai) No. 2001-235772 Official Gazette (Literature 2).
Literature 1 discloses a Raman amplification repeater which captures part of Raman-amplified output power and utilizes the light thus captured to control output power automatically, thereby eliminating the need of using a variable light attenuator for gain control.
Literature 2 discloses a Raman amplification repeater which introduces inspection light into a transmission line, receives the back scattered light generated while the inspection light propagates through the line, and then controls the power of excitation light based on the back scattered light it has received.
These conventional Raman amplification repeaters described above have a problem that the power of excitation light cannot be controlled externally after the transmission system is established.
There is also a problem that the signal bandwidth of signal light to be transmitted is defined in a narrow range, since the center wavelength of excitation light that is input into a transmission line fiber is set in a fixed manner.